The present invention relates to a developing device and to an image forming apparatus equipped with the developing device.
Presently, in an image forming apparatus of a certain type employing an electrophotographic system, there is used a developing method wherein two-component developer composed of toner and carrier, for example, is used, toner which has been subjected to frictional electrification by a proper stirring means is conveyed to a developing area, and the toner is stuck to an electrostatic latent image formed on a latent image carrier, for forming a toner image. In the developing method using the two-component developer of this kind, fresh toner is replenished, depending on an amount of toner consumption, for keeping toner concentration in the developer to be constant.
In the image forming apparatus of this kind, images are required to be of high image quality, and for this requirement, toner with small particle size has come to be used, and this trend of a small particle size of toner has made a carrier to be of a small particle size.
While it is possible to form images with high image quality by using toner with a small particle size and carrier with a small particle size, fluidity of the developer is lowered and it becomes difficult to stir the developer sufficiently, resulting in a problem that the toner newly replenished is supplied to the developing area without becoming the state to have prescribed amount of electrostatic charges, and photographic fog and toner scattering are easily caused.
The problem stated above is caused conspicuously in the case of outputting images having high printing rate continuously, for example, when forming color images.
On the other hand, in recent years, with an elevation of social consciousness for environmental protection, conservation of natural resources has been achieved including toner recycling.
However, the toner collected after being used once for development is inferior to unused toner in terms of charging characteristics, and photographic fog and toner scattering caused by insufficient amount of electrostatic charges are easily caused, which is a problem.
For the problem mentioned above, there have been proposed many technologies for improving efficiency of stirring developer, and for example, a stirring means is made to be one with the structure having high capacity of stirring developer, by providing a member that functions as an auxiliary stirring member (Japanese TOKKAIHEI Nos. 07-013420, 09-166918 and 09-288412).
In the aforementioned technologies, however, cyclic conveyance speed of developer in a developing unit is lowered in all of the technologies, though stirring efficiency for developer is improved, resulting in that toner concentration of developer to be supplied to a developer carrier for the direction of a rotation axis of the developer carrier becomes uneven, and in particular, when images with high printing rate are outputted continuously, uneven density of images tends to be caused and it is difficult to form images with high image quality surely, which is a problem.
When intending to prevent occurrence of photographic fog and toner scattering by improving stirring power for developer for the purpose of making replenished fresh toner to be in the state of having sufficient amount of charges, as stated above, there is a problem that image density unevenness caused by a decline of transportability of developer tends to occur. After all, it is difficult to solve simultaneously two problems including occurrence of photographic fog and toner scattering and occurrence of image density unevenness, which is the real situation.